/Engine/Plugins/Runtime/XRBase/Source/XRBase/Public/HeadMountedDisplayBase.h
Return a FTexture2DRHIRef pointing to a texture that can be composed on top of each eye without applying reprojection to it. Return nullptr to render head locked stereo layers into the same render target as other layer types, in which case InOutViewport must not be modified.
virtual FTexture2DRHIRef GetOverlayLayerTarget_RenderThread ( int32 ViewIndex, FIntRect & InOutViewport )